Breakout protection for crystal growers

ABSTRACT

A CHAMBER FORMED AROUND A HEATED SUSCEPTOR AND CRUCIBLE IN A CRYSTAL GROWER IS CONSTRUCTED TO PROVIDE PROTECTION AGAINST MOLTEN SILICON IN THE EVENT OF BREAKAGE OF THE SUSCEPTOR OR CRUCIBLE. THE BOTTOM OF THE CHAMBER HAS A LAYERED CONSTRUCTION. A FLUID COOLED METALLIC PLATE FORMS THE LOWERMOST LAYER OF THE CHAMBER. THE PLATE IS COVERED BY A LAYER OF SOLID GRAPHITE AND THE SOLID GRAPHITE IS COVERED BY A LAYER OF FELT. IN THE EVENT OF A &#34;BREAKOUT&#34; OF MOLTEN SILICON, THE GRAPHITE FELTS ABSORBS THE MOLTEN MATERIAL AND THE SOLID GRAPHITE TRANSFERS HEAT FROM THE GRAPHITE FELT TO THE FLUID COOLED PLATE, THUS RAPIDLY COOLING THE MASS OF MOLTEN MATERIAL TO SOLIFIFY IT AND PREVENT FURTHER SPREADING THEREOF.   ADDITIONALLY, A TEMPERATURE-SENSING PASSAGEWAY LEADING TO THE SUSCEPTOR IS CAPPED AT THE SUSCEPTOR END THEREOF WITH A DISC OF SILICON-RESISTANT GRAPHITE TO PREVENT MOLTEN SILICON FROM RUNNING INTO THE PASSEWAY.

17, 1972 R. E. REUSSER BREAKOUT PROTECTION FOR CRYSTAL GROWERS Oct.

2 Sheets-Sheet 1 Filed Dec. 28, 1970 INVENTOR .EREUSSER ATTORNEY,

Oct. 17, 1972 R. E. REUSSER BREAKOUT PROTECTION FOR CRYSTAL GROWERS 2 Sheets-Sheet 2.

Filed Dec. 28, 1970 FELT GRA PH/TE COOLING LIQUID FOR METAL BASE United States Patent Office 3,698,872 Patented Oct. 17, 1972 US. Cl. 23-273 SP 1 Claim ABSTRACT OF THE DISCLOSURE A chamber formed around a heated susceptor and crucible in a crystal grower is constructed to provide protection against molten silicon in the event of breakage of the susceptor or crucible. The bottom of the chamber has a layered construction. A fluid cooled metallic plate forms the lowermost layer of the chamber. The plate is covered by a layer of solid graphite and the solid graphite is covered by a layer of felt.

In the event of a breakout of molten silicon, the graphite felts absorbs the molten material and the solid graphite transfers heat from the graphite felt to the fluid cooled plate, thus rapidly cooling the mass of molten material to solidify it and prevent further spreading thereof.

Additionally, a temperature-sensing passageway leading to the susceptor is capped at the susceptor end thereof I with a disc of silicon-resistant graphite to prevent molten silicon from running into the passageway.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to protection against adverse effects of molten material in crystal growers in the event of susceptor or crucible breakage.

Description of the prior art When Czochralski-type crystal growers are used to grow crystals of semiconductor material, such as silicon, the temperature within a crucible is very high, approximately 1420 C. Additionally, the molten silicon is an extremely reactive substance. Because of these conditions, the molten silicon is usually contained within a crucible formed of quartz supported in a graphite susceptor.

Quartz crucibles often have latent defects which sometimes manifest themselves as a breakout when the silicon contained within the crucible is heated to a molten state. In the event of such a breakout, molten silicon will flow through the crucible and contact the graphite susceptor. Graphite at a high temperature, such as 1420 C., will not contain molten silicon. Thus, a failure of a quartz crucible will result in the breakout progressing through the susceptor.

Another condition that occasionally occurs is that the silicon may freeze within the crucible. Silicon has a volume change of approximately 9% upon freezing and this very substantial volume change will cause stressing of the graphite susceptor. Such a stressed graphite susceptor may fail with the result that molten silicon will be released through a breakout.

Unconstrained molten silicon will destroy virtually any machine component with which is comes into contact. In a typical Czochralski-type crystal grower, a substantial portion of the operating mechanism is positioned below the susceptor and crucible, thus, if a breakout in the crucible or susceptor occurs, the molten silicon will flow down over the machine components below and cause substantial damage to the crystal growers.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a system to protect various components of a crystal grower against the adverse effects of molten material in the event that a crucible or susceptor breaks and releases such molten material.

This object is achieved by providing a protective covering formed of a layer of graphite felt and a layer of solid graphite over the various components.

Further protection is achieved by placing the layer of solid graphite in contact with a fluid-cooled member so that thermal energy is quickly absorbed from any unconstrained molten silicon.

Still further protection is provided by placing caps of silicon-resistant graphite onto passageways which might be subjected to the adverse effects of unconstrained molten silicon.

BRlEF DESCRIPTION OF THE DRAWINGS Other objects and features of the present invention will be more readily understood from the following detailed description of specific embodiments thereof, when read in conjunction with the appended drawings in which:

FIG. 1 is an elevational view of a crystal grower with portions thereof removed to more clearly illustrate a susceptor, crucible and heating arrangement of the crystal grower;

FIG. 2 is an enlarged view of the heating arrangement of the crystal grower of FIG. 1 showing an invention protective chamber surrounding the susceptor and crucible.

DETAILED DESCRIPTION Illustratively, the invention will be described in connection with a crystal grower which is producing a silicon crystal. However, it is to be understood that the invention protection system would function equally well with growers used to produce many other types of crystalline materials.

FIG. 1 is an illustration of a Czochralski-type crystal grower, designated generally by the numeral 20. As in most typical Czochralski-type crystal growers, material from which a crystal is to be grown is held in a molten state within a heated crucible 22. A crystal seed 24 is held in the end of a seed shaft 26. The fre end of the seed 24 is touched to the surface of the molten material in the crucible 22 while the crucible and the seed shaft 26 are rotated in opposite directions. The seed shaft 26 is held in the end of a seed shaft 26. The free end of the molten material and a single crystal 28 is thus grown.

Operation of at least one type of Czochralski crystal grower is described in patent application Ser. No. 60,052, filed on July 31, 1970 in the names of J. J. Czeck and R. E. Reusser and assigned to the assignee of record of this application.

In FIG. 2, the heating arrangement of the crystal grower 20 surrounding the crucible 22 is shown in more detail. The crucible 22 is supported in a graphite sus ceptor 30. The susceptor 30 is supported on a graphite pedestal 34 which, in turn, is mounted on a metallic shaft 36.

The shaft 36 provides a connection to a raising-androtating mechanism, designated generally by the numeral 38, see FIG. 1. The mechanism 38 provides a means for rotating the susceptor 30 about its axis and raising and lowering the susceptor in order to provide for proper heating thereof.

The susceptor 30 is held within a graphite heater 40. The heater 40 is a resistance-type heater connected to a source of 3 phase power through three water-cooled electrodes 42.

The crucible 22, susceptor 30 and heater 40 are all contained within an insulated chamber designated generally by the numeral 44. The chamber 44 is provided with fiuid cooled metallic side walls 46 and a fluid cooled metallic bottom plate 48. Within the chamber 44 there is an insulating member, designated generally by the numeral 50, which is formed of a metallic support elment 52 and an insulating element 54 formed of graphite felt.

The bottom plate 48 is covered by a cup-shaped member 58 of solid graphite. Overlying the member 58 is a layer 60 of graphite felt. A suitable material for the graphite member 58 is available from Met-Bay Inc. in Bay City, Mich., and is sold under a trade designation of MB-150. Graphite felt suitable for the formation of the layer 60 can be obtained from Union Carbide, Carbon Products Division, New York, N.Y.

The bottom plate 48, the graphite member 58 and the graphite felt layer 60 are each provided with apertures through which the shaft 36 and the electrodes 42 extend into the chamber 44. Quartz insulators 62 are provided around the electrodes 42 and the shaft 36. The quartz insulator 62 around the shaft 36 is actually large enough to accommodate the graphite pedestal 34.

The structure of the chamber 44 provides substantial protection against the hazards which would result from any breakout of molten silicon from the crucible 22. In the event of such a breakout, the graphite felt layer 60 soaks up the molten silicon and prevents further spreading thereof. The solid graphite member 58 overlying the layer 60 operates as a highly eflicient heat-conduction path to the fluid-cooled bottom plate 48.

The plate 48 is substantially at room temperature throughout the crystal growing process. Because of the relative coolness of the bottom plate 48, the graphite member 58 is also held at a relatively low temperature. The thermal conductivity of graphite is a function of the temperature of the graphite and graphite is particularly conductive when it is cooled. Thus, the member 58 in con tact with the plate 48 provides a very efiicient heat-sinking system to draw away the heat from any molten silicon in contact with the layer 60 of graphite felt.

The combination of these three elements: the layer 60, the member 58 and the plate 48, provides a system in which molten silicon is prevented from spreading and is rapidly cooled to a solid state. Once the silicon is solid of course, the risk of damage from a breakout is substantially eliminated.

Since the molten silicon is contained and solidified within the chamber 44, there is virtually no risk of silicon running down over the very complex and expensive portion of the mechanism 38. Thus, the risk of destroying an entire crystal grower because of a breakout of molten silicon is eliminated by the use of the inventive protection scheme.

The protection against silicon breakout is further enhanced by the presence of the quartz insulator 62 surrounding the electrodes 42 and the shaft 36.

While the protective combination of graphite felt and solid graphite has been described as a bottom of a chamber, it should be realized that this inventive combination of material can be used as a covering over any machine components where protection against the adverse effects of unconstrained molten silicon is desired.

The protective covering of graphite felt and solid graphite has also been described and illustrated with the solid graphite being in direct contact with a fluid-cooled member so that heat is rapidly absorbed from the molten crystal material. Such contact with a fluid-cooled member is highly desirable where large masses of molten crystal material are held in the crucible. Crucibles, at times, hold as much as 10 kilograms of silicon. A breakout of such a large mass of crystal material would saturate the graphite felt and would flow around the felt after saturation were it not for the fact that the heat is rapidly absorbed by the fluid-cooled member.

If the mass of molten crystal material is relatively low, it is possible to achieve the desired breakout protection without a need for a fiuid-cooled member to carry away heat. A small mass of crystal material can be absorbed completely into the graphite felt and would thus be prevented from further spreading.

It should be noted that the chamber 44 is provided with a large vacuum port 64. The port 64 is used to provide roughing vacuum within the chamber 44 prior to introduction of an atmosphere in which the crystal 28 is grown. Typically, growing of silicon crystal by the Czochralski process takes place within a helium or argon atmosphere at a pressure of approximately six p.s.i.g. The roughing vacuum is extremely desirable because of the presence of graphite felt within the chamber 44. Graphite felt, of course, has extremely large surface areas and, consequently, adsorption of air and water vapor occurs. The roughing vacuum eliminates the undesirable air and water vapor from the graphite felt. The insulating member 50 is provided with slots 65 therein to further facilitate removal of the undesired air and water vapor during the outgassing operation.

A further degree of protection against silicon breakout is provided by a graphite cap 66 placed at the susceptor end of a passageway 68 extending through the pedestal 34 and shaft 36. The cap 66 is formed of a siliconresistant grade of graphite available from Met-Bay, Inc. in Bay City, Mich., under the trade designation MB214. This grade of silicon will withstand contact with molten silicon without adverse effects. The passageway 68 permits a conventional radiation detector 70, such Model 8875-C available from Leeds and Northrup Company, North Wales, Pa. (FIG. 1) to be mounted remotely from the susceptor 30 while still being capable of sensing the temperature of the susceptor. The cap 66 will radiate energy with approximately the same characteristics as the susceptor 30. Thus, the cap 66 does not interfere with the temperature sensing operation of the detector 70. The silicon resistant cap 66 will prevent a flow of molten silicon into the passageway 68 in the event of a breakout.

Although certain embodiments of the invention have been shown in the drawings and described in the specification, itis to be understood that the invention is not limited thereto, is capable of modification and can be arranged without departing from the spirit and scope of the invention.

What is claimed is:

1. Apparatus for growing silicon crystals which comprises:

a heated crucible for supporting a molten mass of the silicon; and

a chamber surrounding said crucible, the chamber having a bottom formed of a combination of a first layer of absorptive graphite felt capable of withstanding the temperature of the molten silicon, overlying and contacting a second layer of high density graphite having an annular lip in surrounding relationship to the graphite felt and which is cooled to increase its thermal conductivity and to make it capable of Withstanding the temperature of the molten silicon, said second layer overlying, contacting and being cooled by a liquid-cooled metallic plate whereby the layered bottom of the chamber provides a means for absorb- 5 ing and cooling the molten silicon in the event of 2,975,036 3/1961 Taylor et al 23-273 breakage of the crucible, the chamber having a jack- 3,002,320 10/1961 Theuerer 23-273 eted liquid cooled surrounding side Wall. 3,432,753 3/1969 Horn 23-301 3,499,736 3/1970 Zwaneburg 23-273 References clted I 5 3,551,115 12/1970 Jamieson et a1. 23-273 UNITED STATES PATENTS JR-, Primary Examiner 2,851,341 9/1958 Weiss 23-301 R. T. FOSTER, Assistant Examiner L-566-PT PatencNo. 35,698,872 Dated October 17, 1972 lnventor(s) RAYMOND E REUSSER it is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 21 (SpecO p. 1, line 10) "felts" I should be felt--;

line 64 (Spec. p. 1, line 29) "is" should be --it--,

Column 2, line 52 (Spec. p, 3, line 9) "free" should be -free--;

line 56 (Spec. p. 3, line 12) the passage "held in the end of a seed shaft 26. 'Ilhe free end" should be -slowl y Withdrawn upwardly from the surface".

Signed and sealed this 6th day of March 1973.

(SEAL) Attest:

V EDWARD M,FLETCHER,JR, ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

